The present invention relates to heat-sealable compositions for use in films and laminates. In particular, the invention relates to combinations of ethylene-unsaturated ester copolymer and resin. The combinations exhibit excellent heat sealing and other physical properties without degrading processability and may be used to manufacture a variety of items including laminated objects and other articles employing a heat seal.
Many articles of manufacture are sealed within a protective coating. Some of these coatings are heat-sealed and some are thermally laminated. The heat seals are generally formed by welding two separate portions of the coating or layer (generally either a film or a laminate) together. The sealing layers may form part of a mono- or multi-layer film or laminate, which may or may not comprise additional components and/or additives.
There are several important characteristics of a good heat-sealable polymer. Two important characteristics are the seal initiation temperature (SIT) and maximum seal strength temperature (MSST), which are generally defined by viewing a hot tack strength curve. SIT is the minimum temperature required to develop an acceptable strength of the heat seals. In many cases, the acceptable hot tack strength is equal to or greater than 0.7 N/cm and may depend on the choice of materials and desired minimum hot tack strength.
MSST is the temperature at which the hot tack strength reaches the maximum level, and more importantly, the failure mode is tearing or inseparable bond formation. Thus, heat-sealing temperatures above the seal initiation temperature result in heat seals with considerable and measurable seal strength. Lower heat seal initiation temperatures are desirable during commercial production. Lower temperatures allow higher production rates because the polymer(s) need not be heated to as great a temperature to make the seal(s).
One of the limiting factors controlling the productivity (number of packages/unit time) in commercial packaging lines using heat sealing techniques is the time required to transfer heat to the interface, melt the sealing layers, and cool down, thereby achieving a desired temperature for sealing. Lower SITs and MSSTs require shorter times to transfer the heat to the interface for sealing the surfaces. Also, cooling of the seal to attain adequate strength will be faster. Qualitatively, every 10° C. decrease in the SIT and/or the MSST may result in 30% improvement in the productivity. For applications requiring inseparable bond formation or tearing failure mode, the sealing is usually conducted at temperatures well above the SIT or near the MSST. At these temperatures the polymers are often molten. However, there may be applications where lower seal strength is desirable and peeling or peeling and tearing is required. In such cases, the sealing may be conducted between the SIT and the MSST, and productivity may be significantly increased if good sealing performances are obtained at temperatures closer to the SIT.
A third important sealing characteristic is the cohesive strength during the cooling stage before solidification of a heat seal. Immediately after each seal is formed and as it cools, the sample can be torn apart, and the strength of the seal is measured—this strength is known as hot tack strength. Increased hot tack strength is generally preferred in most applications, particularly in vertical form fill and seal packaging lines.
A fourth important characteristic is the hot tack strength plateau (HTP), which is the range of temperatures acceptable for forming a seal of suitable strength before the sealing polymer cools. The HTP determines the acceptable range of operating temperatures and conditions, such as packaging line speed, where seal strength remains essentially constant or above a certain predetermined value set by the end use application. The acceptable range of operating temperatures depends on the polymers being used. If the operating temperature is too high, the molten polymer does not have time to cool and develop cohesive strength, and the hot tack strength begins to decrease. It is often difficult or impossible to maintain commercial sealing equipment at exactly the same temperature throughout a commercial sealing run. Thus, a broader hot tack strength plateau makes it easier to operate within a given temperature range to ensure that all heat seals made will have acceptable strength. In many cases, the processors determine a predetermined minimum hot tack strength value and operate the sealing equipment at temperatures that will achieve the predetermined minimum hot tack strength value.
The interplay between these sealing characteristics is important to manufacturers, especially in laminate applications. A low SIT allows a manufacturer to use less heat and/or pressure to form a seal, thereby achieving faster production and increased energy savings. An increased hot tack strength yields stronger and more integral seals. A longer HTP provides a strong seal over an increased temperature range and improves process efficiency. Increased hot tack strength prevents the failure, or opening, of a seal at higher processing speeds. All of these in combination provide a manufacturer with the ability to increase line speed of a given process and to broaden the operating window. Manufacturers will benefit from increased production rates and the cost savings associated with operating a process with significantly lower temperatures. End user benefit with better seals in packages.
Various types of polymers are used to form articles that may be joined together or sealed by the application of heat and/or pressure. The polymers or combinations of polymers selected in the sealing layer of the films or laminates are chosen because they provide a strong integral seal (or in certain cases peelability). Occasionally, the sealing layer can form the entire film or may be part of a multilayer film and can be co-extruded with the same or different polymers.
In some applications, ethylene vinyl acetate (EVA) is used alone to form a heat-sealed film in co-extrusion and in laminates because it combines good processability in blown film processes (melt strength, bubble stability) and in extrusion coating processes (draw down, necking), the required compatability with the packed product, and a low melting point. However, EVA has limited thermal stability, which is generally a function of the vinyl acetate comonomer content. EVA copolymers have excellent sealing performances relative to seal strength and low seal initiation temperature. Unfortunately, EVA copolymers generally have relatively lower hot tack strength and narrower HTPs than other polymers such as ethylene acrylic acid (EAA) and ionomers. In vertical form fill and seal packaging lines, only a limited amount of EVA copolymers are used because of its decreased hot tack strength performance. To date, solving these problems has caused the degradation of one or more of the other desired properties. Therefore, a need exists for improvements in EVA copolymer laminate applications that exhibit improved heat sealing while maintaining other desirable physical properties.
There have been prior disclosures of ethylene unsaturated esters and resin combinations, for example, U.S. Pat. No. 4,497,941, U.S. Pat. No. 4,452,835, JP09227840, U.S. Pat. No. 3,625,727, JP Kokai (A) 58-47038, U.S. Pat. No. 5,362,792, JP 51114480, and EP 78122. These disclosures, however, do not appear to recognize that a selection of compatible ethylene unsaturated esters and resins can yield improved processing characteristics, such as increased maximum hot tack strength, lower initial sealing temperatures, and/or extension of the hot tack plateau, while avoiding significant impairment of other key processing parameters.
Other background references include EP 0 369 705 A, GB 2 138 008 A, U.S. Pat. No. 6,376,095 B 1, and GB 1 377 206 A.